1. Field of the Invention
The present invention relates to a light-emitting element driving control circuit.
2. Description of the Related Art
In order to efficiently drive an LED (Light Emitting Diode), which is recently used in various electronic equipment, an LED driving control circuit employing a switching control method might be used (See Japanese Patent Laid-Open Publication No. 2006-230133, for example.)
FIG. 4 is an example of the LED driving control circuit for controlling driving of a white LED for illumination. An LED driving control circuit 100 performs switching for an NMOS transistor 300 to control a driving current Is of white LEDs 310 to 319 (hereinafter referred to as LEDs 310 to 319.) The LED driving control circuit 100 includes a pulse generation circuit 200, a comparator 210, a reference voltage circuit 220, and an SR flip-flop 230.
The pulse generation circuit 200 generates an output signal Vp including a pulse of a high level (hereinafter referred to as H level) in every predetermined cycle TA.
The comparator 210 detects whether or not the driving current Is has reached a predetermined current value I1. Specifically, the comparator 210 compares a detection voltage Vs, which is generated at one end of a detection resistor 340 and generated according to a current value of the driving current Is, with a reference voltage Vref of a reference voltage circuit 220. When the detection voltage Vs becomes higher than the reference voltage Vref, it is considered that the driving current Is has reached the predetermined current value I1, and the comparator 210 changes an output signal Vc from a low level (hereinafter referred to as L level) to the H level.
The SR flip-flop 230 changes a Q output to the H level to turn on the NMOS transistor 300 when the output signal Vp from the pulse generation circuit 200 is changed to the H level. On the other hand, the SR flip-flop 230 changes the Q output to the L level to turn off the NMOS transistor 300 when the output signal Vc of the comparator 210 is change to the H level.
A change of the driving current Is will now be described referring to an upper side of a timing chart shown in FIG. 5. First, when the output signal Vp is changed to the H level at a time T0, the Q output of the SR flip-flop 230 is changed to the H level, and thus, the NMOS transistor 300 is turned on. As a result, the driving current Is is increased at a speed corresponding to an inductance L of an inductor 320 and a level of a power supply voltage VDD. Since the driving current Is is supplied to the detection resistor 340 through the NMOS transistor 300 which has been turned on, the detection voltage Vs is also raised according to the increase of the driving current Is. When the current value of the driving current Is becomes equal to the predetermined current value I1 at a time T1, that is, when the detection voltage Vs becomes equal to the reference voltage Vref, the output signal Vc of the comparator 210 is changed to the H level, and thus, the Q output of the SR flip-flop 230 is changed to the L level. As a result, the NMOS transistor 300 is turned off, and the energy stored in the inductor 320 is released through a loop of the LEDs 310 to 319, the inductor 320, and a diode 330. The energy stored in the inductor 320 is released by the driving current Is at a speed corresponding to the inductance L and respective levels of forward voltages of the LEDs 310 to 319 and the diode 330. As above, the predetermined current value I1 is the maximum value of the driving current Is, and the LED driving control circuit 100 controls the NMOS transistor 300 so that the driving current Is does not exceed the maximum value. Since the driving current Is is decreased at the time T1, the output signal Vc of the comparator 210 is changed to the L level.
At a time T3 at which one cycle of the output signal Vp has elapsed from the time T0, the output signal Vp of the pulse generation circuit 200 is changed to the H level, and thus, the NMOS transistor 300 is turned on and the driving current Is is increased as in the case with the time T0. In this way, a change from the time T0 to the time T3 is repeated at the time T3 and thereafter. Since the driving current Is is changed in the cycle TA, an average value of the driving current Is is a predetermined value, and thus, the LEDs 310 to 319 are driven by a constant current. If the power supply voltage VDD is increased and the speed of increase of the driving current Is is increased, for example, a period of ON-time of the NMOS transistor 300 is reduced, but a cycle during which the transistor 300 is turned on is not changed. That is, the LED driving control circuit 100 is a switching circuit, which employs a pulse-width modulation method, for changing a pulse width of ON-time when the NMOS transistor 300 is turned on in the cycle TA.
As described above, the LED driving control circuit 100 performs switching for the NMOS transistor 300 in the cycle TA so that the LEDs 310 to 319 are driven by a constant current. As a result, the cycle of the driving current Is also becomes equal to the cycle TA similarly to a switching cycle.
However, as shown in a lower side of the timing chart in FIG. 5, when the driving current Is, which is changed in the cycle TA before the time T0, is reduced due to transitional fluctuations of the power supply voltage VDD, for example, the cycle of the driving current Is does not become equal to the cycle TA even if the power supply voltage VDD is not changed from a desired level at the time T0 and thereafter. Specifically, when the NMOS transistor 300 is turned on at the time T0, the actual driving current Is indicated by a solid line is increased at a speed equivalent to the speed of increase of the driving current Is in the cycle TA indicated by a dotted line, that is, the speed corresponding to the inductance L of the inductor 320 and the level of the power supply voltage VDD. As a result, the actual driving current Is reaches the current value I1 at the time T2 later than the above-mentioned time T1. Then, when the NMOS transistor 300 is turned off at the time T2, the actual driving current Is is decreased at a speed equivalent to the speed of decrease of the driving current Is in the cycle TA, that is, the speed corresponding to the inductance L and the forward voltage level of the LEDs 310 to 319 and the diode 330. At the time T3 at which the output signal Vp is changed to the H level, the NMOS transistor 300 is turned on, and thus, the actual driving current Is is increased. Since the actual driving current Is at the time T3 is greater in current value than the driving current Is in the cycle TA, the actual driving current Is reaches the current value I1 at a time T4 earlier than a time T5. When the NMOS transistor 300 is turned off at the time T4, the actual driving current Is is decreased until a time T6 at which one cycle of the output signal Vp has elapsed from the time T3. The actual driving current Is at the time T6 is much lower in current value than the driving current Is in the cycle TA. Therefore, even if the NMOS transistor 300 is turned on at the time T6, the actual driving current Is will not reach the current value I1 by a time T7 at which one cycle of the output signal Vp has elapsed from the time T6, but reaches the current value I1 at a time T8 within a period from the time T7 to the time at which one cycle of the output signal Vp has elapsed.
As described above, even if the switching cycle TA of the NMOS transistor 300, the speed of increase and the speed of decrease of the driving current Is, and the current value I1 for detecting the maximum value of the driving current Is are constant, the cycle of the actual driving current Is may not be equal to the cycle TA. That is, when the NMOS transistor 300 is turned on in the cycle TA and the maximum value of the driving current Is is detected to control the driving current Is as mentioned above, sub-harmonic oscillation which oscillates in a cycle longer than the cycle TA may be generated.